sysprofd: add support for unwinding without frame pointers

This provides a new sysprof-live-unwinder subprocess that runs as root to
allow accessing all processes on the system via /proc/$pid/. It is spawned
by sysprofd with various perf event FDs and a FD to write captures to.

Ideally the capture_fd is something that will naturally error if the client
application crashes (such as a socketpair() having the peer close). This
is not enforced but encouraged. Additionally, an event_fd is used to allow
the client application to signal the live-unwinder to exit.

Unwinding is performed by looking at the modules loaded into the target
pid and using libdwfl to access DWARF/CFI/etc state machinery. Stack data
does not touch the disk as it exists in a mmap buffer from perf and is
then translated into a callchain and sent to the Sysprof client.

Unwinding occurs as normal post-mortem though is improved through the use
of debuginfod to locate the appropriate symbols.

src/sysprofd/ipc-unwinder-impl.c

@@ -0,0 +1,247 @@
+/*
+ * ipc-unwinder-impl.c
+ *
+ * Copyright 2024 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#define G_LOG_DOMAIN "ipc-unwinder-impl"
+
+#include "config.h"
+
+#include <errno.h>
+
+#include <signal.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
+
+#include <glib/gstdio.h>
+
+#include <polkit/polkit.h>
+
+#include "ipc-unwinder-impl.h"
+
+struct _IpcUnwinderImpl
+{
+  IpcUnwinderSkeleton parent_instance;
+};
+
+static void
+child_setup (gpointer data)
+{
+  prctl (PR_SET_PDEATHSIG, SIGKILL);
+}
+
+static gboolean
+ipc_unwinder_impl_handle_unwind (IpcUnwinder           *unwinder,
+                                 GDBusMethodInvocation *invocation,
+                                 GUnixFDList           *fd_list,
+                                 guint                  stack_size,
+                                 GVariant              *arg_perf_fds,
+                                 GVariant              *arg_event_fd)
+{
+  g_autoptr(GSubprocessLauncher) launcher = NULL;
+  g_autoptr(GSubprocess) subprocess = NULL;
+  g_autoptr(GUnixFDList) out_fd_list = NULL;
+  g_autoptr(GPtrArray) argv = NULL;
+  g_autoptr(GError) error = NULL;
+  g_autofd int our_fd = -1;
+  g_autofd int their_fd = -1;
+  g_autofd int event_fd = -1;
+  GVariantIter iter;
+  int capture_fd_handle;
+  int pair[2];
+  int next_target_fd = 3;
+  int perf_fd_handle;
+  int cpu;
+
+  g_assert (IPC_IS_UNWINDER_IMPL (unwinder));
+  g_assert (G_IS_DBUS_METHOD_INVOCATION (invocation));
+  g_assert (!fd_list || G_IS_UNIX_FD_LIST (fd_list));
+
+  if (stack_size == 0 || stack_size % sysconf (_SC_PAGESIZE) != 0)
+    {
+      g_dbus_method_invocation_return_error_literal (g_steal_pointer (&invocation),
+                                                     G_DBUS_ERROR,
+                                                     G_DBUS_ERROR_INVALID_ARGS,
+                                                     "Stack size must be a multiple of the page size");
+      return TRUE;
+    }
+
+  if (fd_list == NULL)
+    {
+      g_dbus_method_invocation_return_error_literal (g_steal_pointer (&invocation),
+                                                     G_DBUS_ERROR,
+                                                     G_DBUS_ERROR_FILE_NOT_FOUND,
+                                                     "Missing perf FDs");
+      return TRUE;
+    }
+
+  launcher = g_subprocess_launcher_new (0);
+  argv = g_ptr_array_new_with_free_func (g_free);
+
+  g_ptr_array_add (argv, g_strdup (PACKAGE_LIBEXECDIR "/sysprof-live-unwinder"));
+  g_ptr_array_add (argv, g_strdup_printf ("--stack-size=%u", stack_size));
+
+  if (-1 == (event_fd = g_unix_fd_list_get (fd_list, g_variant_get_handle (arg_event_fd), &error)))
+    {
+      g_dbus_method_invocation_return_gerror (g_steal_pointer (&invocation), error);
+      return TRUE;
+    }
+
+  g_ptr_array_add (argv, g_strdup_printf ("--event-fd=%u", next_target_fd));
+  g_subprocess_launcher_take_fd (launcher, g_steal_fd (&event_fd), next_target_fd++);
+
+  g_variant_iter_init (&iter, arg_perf_fds);
+
+  while (g_variant_iter_loop (&iter, "(hi)", &perf_fd_handle, &cpu))
+    {
+      g_autofd int perf_fd = g_unix_fd_list_get (fd_list, perf_fd_handle, &error);
+
+      if (perf_fd < 0)
+        {
+          g_dbus_method_invocation_return_gerror (g_steal_pointer (&invocation), error);
+          return TRUE;
+        }
+
+      g_ptr_array_add (argv, g_strdup_printf ("--perf-fd=%d:%d", next_target_fd, cpu));
+      g_subprocess_launcher_take_fd (launcher,
+                                     g_steal_fd (&perf_fd),
+                                     next_target_fd++);
+    }
+
+  g_subprocess_launcher_set_child_setup (launcher, child_setup, NULL, NULL);
+
+  if (socketpair (AF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0, pair) < 0)
+    {
+      int errsv = errno;
+      g_dbus_method_invocation_return_error_literal (g_steal_pointer (&invocation),
+                                                     G_IO_ERROR,
+                                                     g_io_error_from_errno (errsv),
+                                                     g_strerror (errsv));
+      return TRUE;
+    }
+
+  our_fd = g_steal_fd (&pair[0]);
+  their_fd = g_steal_fd (&pair[1]);
+
+  out_fd_list = g_unix_fd_list_new ();
+  capture_fd_handle = g_unix_fd_list_append (out_fd_list, their_fd, &error);
+
+  if (capture_fd_handle < 0)
+    {
+      g_dbus_method_invocation_return_gerror (g_steal_pointer (&invocation), error);
+      return TRUE;
+    }
+
+  g_ptr_array_add (argv, g_strdup_printf ("--capture-fd=%d", next_target_fd));
+  g_subprocess_launcher_take_fd (launcher, g_steal_fd (&our_fd), next_target_fd++);
+
+  g_ptr_array_add (argv, NULL);
+
+  if (!(subprocess = g_subprocess_launcher_spawnv (launcher, (const char * const *)argv->pdata, &error)))
+    {
+      g_dbus_method_invocation_return_gerror (g_steal_pointer (&invocation), error);
+      return TRUE;
+    }
+
+  ipc_unwinder_complete_unwind (unwinder,
+                                g_steal_pointer (&invocation),
+                                out_fd_list,
+                                g_variant_new_handle (capture_fd_handle));
+
+  g_subprocess_wait_async (subprocess, NULL, NULL, NULL);
+
+  return TRUE;
+}
+
+static void
+unwinder_iface_init (IpcUnwinderIface *iface)
+{
+  iface->handle_unwind = ipc_unwinder_impl_handle_unwind;
+}
+
+G_DEFINE_FINAL_TYPE_WITH_CODE (IpcUnwinderImpl, ipc_unwinder_impl, IPC_TYPE_UNWINDER_SKELETON,
+                               G_IMPLEMENT_INTERFACE (IPC_TYPE_UNWINDER, unwinder_iface_init))
+
+static gboolean
+ipc_unwinder_impl_g_authorize_method (GDBusInterfaceSkeleton *skeleton,
+                                      GDBusMethodInvocation  *invocation)
+{
+  PolkitAuthorizationResult *res = NULL;
+  PolkitAuthority *authority = NULL;
+  PolkitSubject *subject = NULL;
+  const gchar *peer_name;
+  gboolean ret = TRUE;
+
+  g_assert (IPC_IS_UNWINDER_IMPL (skeleton));
+  g_assert (G_IS_DBUS_METHOD_INVOCATION (invocation));
+
+  peer_name = g_dbus_method_invocation_get_sender (invocation);
+
+  if (!(authority = polkit_authority_get_sync (NULL, NULL)) ||
+      !(subject = polkit_system_bus_name_new (peer_name)) ||
+      !(res = polkit_authority_check_authorization_sync (authority,
+                                                         POLKIT_SUBJECT (subject),
+                                                         "org.gnome.sysprof3.profile",
+                                                         NULL,
+                                                         POLKIT_CHECK_AUTHORIZATION_FLAGS_ALLOW_USER_INTERACTION,
+                                                         NULL,
+                                                         NULL)) ||
+      !polkit_authorization_result_get_is_authorized (res))
+    {
+      g_dbus_method_invocation_return_error (g_steal_pointer (&invocation),
+                                             G_DBUS_ERROR,
+                                             G_DBUS_ERROR_ACCESS_DENIED,
+                                             "Not authorized to make request");
+      ret = FALSE;
+    }
+
+  g_clear_object (&authority);
+  g_clear_object (&subject);
+  g_clear_object (&res);
+
+  return ret;
+}
+
+static void
+ipc_unwinder_impl_finalize (GObject *object)
+{
+  G_OBJECT_CLASS (ipc_unwinder_impl_parent_class)->finalize (object);
+}
+
+static void
+ipc_unwinder_impl_class_init (IpcUnwinderImplClass *klass)
+{
+  GObjectClass *object_class = G_OBJECT_CLASS (klass);
+  GDBusInterfaceSkeletonClass *skeleton_class = G_DBUS_INTERFACE_SKELETON_CLASS (klass);
+
+  object_class->finalize = ipc_unwinder_impl_finalize;
+
+  skeleton_class->g_authorize_method = ipc_unwinder_impl_g_authorize_method;
+}
+
+static void
+ipc_unwinder_impl_init (IpcUnwinderImpl *self)
+{
+}
+
+IpcUnwinder *
+ipc_unwinder_impl_new (void)
+{
+  return g_object_new (IPC_TYPE_UNWINDER_IMPL, NULL);
+}

src/sysprofd/ipc-unwinder-impl.h

@@ -0,0 +1,34 @@
+/*
+ * ipc-unwinder-impl.h
+ *
+ * Copyright 2024 Christian Hergert <chergert@redhat.com>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+#pragma once
+
+#include "ipc-unwinder.h"
+
+G_BEGIN_DECLS
+
+#define IPC_TYPE_UNWINDER_IMPL (ipc_unwinder_impl_get_type())
+
+G_DECLARE_FINAL_TYPE (IpcUnwinderImpl, ipc_unwinder_impl, IPC, UNWINDER_IMPL, IpcUnwinderSkeleton)
+
+IpcUnwinder *ipc_unwinder_impl_new (void);
+
+G_END_DECLS

src/sysprofd/meson.build

@@ -10,18 +10,24 @@ ipc_service_src = gnome.gdbus_codegen('ipc-service',
          namespace: 'Ipc',
 )
 
+ipc_unwinder_src = gnome.gdbus_codegen('ipc-unwinder',
+           sources: 'org.gnome.Sysprof3.Unwinder.xml',
+  interface_prefix: 'org.gnome.Sysprof3.',
+         namespace: 'Ipc',
+)
+
 sysprofd_sources = [
   'sysprofd.c',
   'ipc-rapl-profiler.c',
   'ipc-service-impl.c',
+  'ipc-unwinder-impl.c',
   'sysprof-turbostat.c',
   'helpers.c',
   ipc_profiler_src,
   ipc_service_src,
+  ipc_unwinder_src,
 ]
 
-pkglibexecdir = join_paths(get_option('prefix'), get_option('libexecdir'))
-
 sysprofd_deps = [
   glib_dep,
   gio_dep,

src/sysprofd/org.gnome.Sysprof3.Unwinder.xml

@@ -0,0 +1,23 @@
+<!DOCTYPE node PUBLIC "-//freedesktop//DTD D-BUS Object Introspection 1.0//EN"
+"http://www.freedesktop.org/standards/dbus/1.0/introspect.dtd">
+<node>
+  <interface name="org.gnome.Sysprof3.Unwinder">
+    <!--
+      Unwind:
+      @stack_size: the size of stacks that are sampled
+      @perf_fds: an array of (perf_fd, CPU number)
+      @event_fd: an event fd to write to for notifying unwinder should exit
+      @capture_fd: (out): a FD that will be written to containing samples
+
+      Unwinding will stop when capture_fd can no longer be written to
+      such as being closed by the consumer of this API.
+    -->
+    <method name="Unwind">
+      <annotation name="org.gtk.GDBus.C.UnixFD" value="true"/>
+      <arg type="u" name="stack_size" direction="in"/>
+      <arg type="a(hi)" name="perf_fds" direction="in"/>
+      <arg type="h" name="event_fd" direction="in"/>
+      <arg type="h" name="capture_fd" direction="out"/>
+    </method>
+  </interface>
+</node>

src/sysprofd/sysprofd.c

@@ -30,9 +30,11 @@
 
 #include "ipc-rapl-profiler.h"
 #include "ipc-service-impl.h"
+#include "ipc-unwinder-impl.h"
 
 #define V3_PATH                 "/org/gnome/Sysprof3"
 #define RAPL_PATH               "/org/gnome/Sysprof3/RAPL"
+#define UNWINDER_PATH           "/org/gnome/Sysprof3/Unwinder"
 #define NAME_ACQUIRE_DELAY_SECS 3
 #define INACTIVITY_TIMEOUT_SECS 120
 
@@ -126,6 +128,7 @@ main (gint   argc,
     {
       g_autoptr(IpcProfiler) rapl = ipc_rapl_profiler_new ();
       g_autoptr(IpcService) v3_service = ipc_service_impl_new ();
+      g_autoptr(IpcUnwinder) unwinder = ipc_unwinder_impl_new ();
 
       g_signal_connect (v3_service, "activity", G_CALLBACK (activity_cb), NULL);
       g_signal_connect (rapl, "activity", G_CALLBACK (activity_cb), NULL);
@@ -133,7 +136,8 @@ main (gint   argc,
       activity_cb (NULL, NULL);
 
       if (g_dbus_interface_skeleton_export (G_DBUS_INTERFACE_SKELETON (v3_service), bus, V3_PATH, &error) &&
-          g_dbus_interface_skeleton_export (G_DBUS_INTERFACE_SKELETON (rapl), bus, RAPL_PATH, &error))
+          g_dbus_interface_skeleton_export (G_DBUS_INTERFACE_SKELETON (rapl), bus, RAPL_PATH, &error) &&
+          g_dbus_interface_skeleton_export (G_DBUS_INTERFACE_SKELETON (unwinder), bus, UNWINDER_PATH, &error))
         {
           for (guint i = 0; i < G_N_ELEMENTS (bus_names); i++)
             {